In the related art, a single stage centrifugal compressor is known as a centrifugal fluid machine (for example, see Patent Literature 1). This centrifugal compressor includes a diffuser passage that allows an impeller, which is attached to a turbine shaft, to communicate with scrolls formed on the discharge side of the impeller and on the outer circumferential side thereof. This diffuser passage is provided with a guiding blade unit that includes a guiding blade. The guiding blade unit protrudes into or retreats from the diffuser passage, depending on its operating mechanism. Specifically, the guiding blade unit retreats from the diffuser passage by the negative pressure in a rear air chamber. On the other hand, the guiding blade unit protrudes into the diffuser passage by being pressed by means of a protruded spring provided in the rear air chamber when the negative pressure therein is released and the air in the diffuser passage flows in through a vent hole that communicates with the rear air chamber. Thus, the centrifugal compressor can enhance efficiency in a low flow area by protruding the guiding blade unit into the diffuser passage, and prevents a decrease in efficiency in a high flow area by retreating the guiding blade unit from the diffuser passage.